Last week I grabbed some swatch book samples for Lee and Rosco filter products from my local production supply house. If you don't have these I would recommend that you get them. The swatches are free and help you choose filter colors and diffusion material without having to just buy a bunch of stuff and try it. The reason I grabbed the swatch books is that I couldn't find any good reading material on the Internet about standard film lighting diffusion material - i.e. 216, 250, opal, soft frost, grid cloth...etc. Most professionals seem to have their selection of what diffusion material to use in which situation and it made me feel like I needed to do a bit of self education.
First, I want to say that I come into this study with no particular preference for brand. Despite similar naming conventions the brands of diffusion and filters have some very significant differences. It was sort of like when I had a sheet of full CTB from Rosco and went into the shop looking for more. They only had Lee, which looked quite a bit different! That taught me to stick with one brand for the particular product I was looking for. The same can be said of diffusion.
So let's start looking at the diffusion materials, shall we? 216 is the catch all, default diffusion material. I once read a recommendation from an ASC member that said that if you don't know what you want, start with 216, then decide if you want more or less diffusion. While Rosco claims a "minimal" color shift, Lee actually rates the color temperature of the material at 6774K, which means it will add blue to your lighting. Also, if you look at the pictures you'll see that Lee is more dense than Rosco (this isn't a good/bad judgment). The density is supposed to be similar to tracing paper, but tough enough to withstand film production.
Another common type of diffusion material is grid cloth. Grid cloth is known for creating a mixture of diffusion and directional light. As you can see in the photos below the cloth is a woven grid which heavily diffuses light, along with a less dense base which allows some directionality. I often hear about other DP's using this as a close proximity diffusion material to emulate diffuse daylight. One thing that I noticed when reviewing the two swatches is that there is a color temperature difference between the two materials. The Lee fabric is slighting cooler and the Rosco material is slightly warmer. You may or may not be able to see the difference on your screen, so I would recommend reviewing the swatches in real life.
Grid cloth comes in a few densities. Another common fabric is light grid cloth. As the name implies, it's a lighter density than full grid cloth. Here the Rosco brand is a bit more diffuse than Lee, however both maintain about the same grid geometry.
Speaking of density, a common use of grid cloth is diffusing light outdoors. The problem is that these industrial fabrics can be quite tough and stiff, so they make noise that will drive your sound person nuts in the wind. For these situations the vendors make silent grid cloth. It tends to be a bit more dense than regular grid cloth, but as you'll see in the photos it's also much more pliable. So when silent grid cloth flaps in the wind it doesn't make as much noise.
A few DP's I know of like to use either quarter grid cloth or half-soft frost for overhead diffusion outdoors. I think it depends on the amount of "raccoon eye" correction you're going for, because sometimes I want a full grid or a silk. The photos below show that Rosco is a bit more dense and has a larger grid pattern than Lee, so the diffusion is going to have a different effect between brands. The dot pattern on half-soft frost is also a bit different, so the two brands have slightly different diffusion characteristics (but I probably wouldn't be able to notice a difference).
I realize this can all be a bit confusing. You're probably thinking what I was thinking at first - "OK, what do I use?" That just has to come from experience and working with a great gaffer. My best advice is to follow what a wise DP once told me - "start with 216 and figure out if you need more or less diffusion."
One thing I've been meaning to do lately is go on another expedition to the fabric store and figure out which of these diffusion materials I can closely approximate with regular fabric. "Rags", as the industry calls them, can be quite expensive. Sometimes you can get something close enough for $10 at a fabric store as long as it doesn't have to be flame retardant or made for the rigors of daily production.